In recent years, heat curable coating materials have been desired that cure at lower temperatures, from the viewpoint of reducing the energy levels used during heating. To meet this demand, heat curable coating materials containing block polyisocyanate compounds in which isocyanate groups are blocked with active methylene compounds such as acetoacetic acid esters and malonic acid diesters, are promising as curing agents because they cure at relatively low temperatures.
Also, conversion of coating materials to aqueous forms has also been desirable in recent years from the viewpoint of reducing environmental pollution by volatilization of organic solvents.
However, when a block polyisocyanate compound is used having isocyanate groups blocked with an active methylene compound for an aqueous coating material, the storage stability has often been reduced. Specifically, if an aqueous coating material containing a block polyisocyanate compound in which isocyanate groups are blocked with an active methylene compound is coated after storage, the adhesion of the coating film that is formed is often reduced. This is believed to be due to dissociation of the active methylene compound in the block polyisocyanate compound during storage of the aqueous coating material, and reaction of the generated isocyanate groups with water in the aqueous coating material resulting in inactivation, whereby the curability of the aqueous coating material after storage is reduced.
In addition, coating materials are generally desired that can form coating films with excellent smoothness and sharpness.
For example, PTL 1 teaches that a block polyisocyanate compound with two different active methylene-based compounds with specific structures as blocking agents can form a crosslinked coating film at 90° C. or below, and has excellent storage stability in the presence of moisture.